1. Field of the Invention
The invention relates to an electrooptical liquid crystal system
which between 2 electrode layers contains a PDLC film comprising a liquid crystal mixture being dispersed in form of microdroplets in an optically isotropic, transparent polymer matrix, PA1 in which one of the refractive indices of the liquid crystal mixture is matched to the refractive index of the polymer matrix, and PA1 which in one of the two switching states has a reduced transmission compared with the other state, essentially independent of the polarization of the incident light. PA1 a high HR and PA1 a Swiss cheese morphology. PA1 X.sup.1 and X.sup.2, independently of one another, are H or F, PA1 Q is CF.sub.2, OCF.sub.2, C.sub.2 F.sub.4, OC.sub.2 F.sub.4 or a single bond, PA1 Y is H, F, Cl or CN, PA1 n is 0, 1 or 2 and PA1 R is alkyl having up to 13 C atoms, in which one or two non-adjacent CH.sub.2 groups can also be replaced by --O-- and/or --CH.dbd.CH--, PA1 a component A containing 0.1-35 wt. % of one or more at least difunctional thiol monomers and/or oligomers PA1 a component B containing 5-95 wt. % of one or more monomers of the ene type, and PA1 a component C containing 2-55 wt. % of one or more oligomers of the ene type and PA1 which between 2 electrode layers contains a PDLC film comprising a liquid crystal mixture forming microdroplets in an optically isotropic, transparent polymer matrix, PA1 in which one of the refractive indices of the liquid crystal mixture is matched to the refractive index of the polymer matrix, PA1 which in one of the two switching states has reduced transmission compared with the other state, essentially independent of the polarization of the incident light, PA1 whose precursor of the PDLC film is selected as stated above. PA1 a component A containing 0.1-35 wt. % of one or more at least difunctional thiol monomers and/or oligomers, PA1 a component B containing 5-95 wt. % of one or more oligomers of the ene type, and PA1 a component C containing 2-55 wt. % of one or more oligomers of the ene type, and PA1 Z is independently from each other a single bond or --CH.sub.2 CH.sub.2 --, PA1 l and m are independently from each other 0 or 1, and ##STR13## denotes 1,4-phenylene, 2-fluoro-1,4-phenyle or 3-fluoro-1,4-phenylene.
2. Description of the Related Art
The preparation of PDLC (=polymer dispersed liquid crystal) films is described, for example, in U.S. Pat. No. 4,688,900, Mol. Cryst. Liq. Cryst. Nonlin. Optic, 157, 1988, 427-441, WO 89/06264 and EP 0,272,585. In the so-called PIPS technology (=polymerization-induced phase separation) the liquid crystal mixture is first homogenously mixed with monomers and/or oligomers of the matrix-forming material; phase-separation is then induced by polymerization. Differentiation must further be made between TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) (Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427) both also being methods to produce PDLC films.
The process of preparation must be controlled very carefully in order to obtain systems with good electrooptical properties. F. G. Yamagishi et al., SPIE Vol. 1080, Liquid Crystal Chemistry, Physics and Applications, 1989, p.24 differentiate between a "Swiss cheese" and a "polymer ball" morphology. In the latter one, the polymer matrix consists of small polymer particles or "balls" being connected or merging into each other while in the Swiss cheese system, the polymer matrix is continuous and exhibits well defined, more or less spherical voids containing the liquid crystal. The Swiss cheese morphology is preferred because it exhibits a reversible electrooptical characteristic line while the polymer ball system shows a distinct hysteresis generally leading to a drastic deterioration of the electrooptical characteristic line when comparing the virgin and the second run.
According to Yamagishi et al., loc. cit., the Swiss cheese morphology is promoted in case the polymerization reaction runs via a step mechanism, and in WO 89/06264 it is pointed out that the step mechanism is favored in case the precursor of the polymer matrix consists of multifunctional acrylates and multifunctional mercaptanes. Another important point which can be influenced by a careful selection of process parameters such as curing temperature, relative concentration and choice of the components of the prepolymer etc., is the mean size of the microdroplets. The mean droplet size generally influences the threshold voltage, the switching times, the on-state transmission and other properties of the PDLC film. Experimental methods for influencing the average droplet size are described, for example, in U.S. Pat. No. 4,673,255 and in J. L. West, Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt., 157, 1988, 427. In U.S. Pat. No. 4,673,255, average drop diameters between 0.1 .mu.m and 8 .mu.m are given, while, for example, a matrix which is based on a glass monolith has pores having a diameter between 15 and 2,000 .ANG.. For the mesh width of the network of PN systems, a preferred range between 0.5 and 2 .mu.m is given in EP 0,313,053.
In PDLC films, one of the refractive indices of the liquid crystal mixture, customarily the ordinary refractive index n.sub.o, is selected in such a way that it more or less coincides with the refractive index n.sub.p of the polymeric matrix. If no voltage is applied to the electrodes, the liquid crystal molecules in the droplets exhibit a distorted alignment, and incident light is scattered at the phase boundary between the polymeric and liquid crystal phases.
On applying a voltage, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the transmitted light. Normally incident light (viewing angle .theta.=0.degree.) now sees an optically isotropic medium and appears transparent.
No polarizers are required for operating PDLC systems, as a result of which these systems have high transmission. PDLC systems provided with active matrix addressing have been proposed on the basis of these favorable transmission properties in particular for projection applications, but in addition also for displays having high information content and for further applications.
The liquid crystal mixtures used for producing PDLC systems have to meet a wide range of demands. One of the refractive indices of the liquid crystal mixture is selected such that it matches with the refractive index of the polymer matrix. The term matching of refractive indices used here covers not only the case n.sub.o (resp. another refractive index of the liquid crystal mixture)--n.sub.p, but also the condition n.sub.o (resp. another refractive index of the liquid crystal mixture)&lt;n.sub.p which is sometimes chosen to reduce off-axis haze and enlarge the view angle as described, for example, in EP 0,409,442.
The liquid crystal mixture preferably has a positive dielectric anisotropy but the use of dielectrically negative liquid crystal mixtures (see, for example, WO 91/01511) or two-frequency liquid crystal mixtures (see, for example, N. A. Vaz et al., J. Appl. Phys. 65, 1989, 5043) is also discussed.
Furthermore, the liquid crystal mixture should have a high clearing point, a broad nematic range, no smectic phases down to low temperatures and a high stability and should be distinguished by an optical anisotropy .DELTA.n and a flow viscosity .eta. which can to optimized with respect to the particular application, and by a high electrical anisotropy.
A series of matrix materials and polymerization processes have hitherto been proposed for producing PDLC system. The PIPS, SIPS and TIPS technologies are described in some detail in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Optics, 157, 1988, 427. The PDLC systems described in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Optics, 157, 1988, 427 are based on an epoxy film, while in EP 0,272,585 acrylate systems are given. The PDLC system of WO 89/06264 is based on multifunctional acrylates and multifunctional thiols, and Y. Hirai et al., SPIE Vol. 1257, Liquid Crystal Displays and Applications, 1990, p.2 describe PDLC system the precursor of the polymer matrix of which being based on monomers and oligomers. Further suitable matrix materials are described, for example, in U.S. 3,935,337, WO 91/13126, EP 0,205,261 and in further references.
Electrooptical systems containing PDLC films can be addressed passively or actively. Active driving schemes employing an active matrix having nonlinear addressing elements integrated with the image point, are especially useful for displays with high information content. The nonlinear elements used for preparing the active matrix type electrode film can have 2 or 3 connections. Examples of elements having 2 connections are a-Si:H diodes (N. Szydlo et al., Proc. 3rd Int. Display Res. Conf., Kobe; SID Los Angeles, 1983, p. 416), NINa-Si:H diodes (Z. Yaniv et al., Conf. Rec. 1985 Intern. Display Research Conference, IEEE, New York, 1985, p. 76), a-Si:H ring diodes (S. Togashi et al., Proceedings of the 4th International Display Research Conference, SEE, Paris, 1984, p. 141), MIM or MSI diodes (metal-insulator-metal, metal-silicon nitrideindium tin oxide; D. R. Baraff et al., Digest SID International Symposium, Los Angeles, 1980, p. 200; M. Suzuki et al., Proceedings of the 6th International Display Research Conference, Japan Display '86, 1986, p. 72) or ZnO varistores (D. E. Castleberry et al., SID '82 Digest, 1982, p. 246). The nonlinear elements having 3 connections are thin film transistors (TFT), of which several types are discussed and which differ in the semiconductor material used (for example a-Si:H, p-Si, CdSe, Te and other materials; see, for example, P. M. Knoll, Displays, Heidelberg 1986, p. 216; T. Nishimura, Mat. Res. Soc. Symp. Proc. 33, (1984) 221; C. S. Bak et al., Mat. Res. Soc, Symp. Proc. 33 (1984) 215; W. G. Hawkins et al., Mat. Res. Soc. Symp. Proc. 33, (1984) 231; M. Matsuura et al., SID 1983 Symposium Digest, 1983, p. 148).
When nonlinear elements having 3 connections are used, only one connection is usually required for the counter electrode, while in the case of active matrix addressings, which are based on elements having 2 connections, the counter electrode is usually also scanned. However, active matrix addressings based on elements having 2 connections and in which only one electrode is scanned have also been proposed (Y. Baron et al., Proceedings of the 6th International Research Conference 1986, Japan Display '86, p. 86), and furthermore the use of TFTs as an element having only 2 connections has also been discussed (C. Hilsum et al., Displays, January 1986 p. 37).
More details on the addressing of liquid crystal displays by an active matrix of nonlinear elements can be found, for example, in A. H. Firester, SID, 1987, Society for information Display Seminar, Seminar 5: Active Matrices for Liquid Crystals, E. Kaneko, Liquid Crystal Displays, KTK Scientific Publishers, Tokyo, Japan, 1987, chapter 6 and 7 or P. M. Knoll, Display, Heidelberg, 1986, p. 216 ff.
When the PDLC system is addressed by means of an active matrix, a further far reaching criterion is added to the requirements listed so far which must be fulfilled by the cured polymer and the liquid crystal mixture being embedded in microdroplets. This is related to the fact that each image point represents a capacitive load with respect to the particular active nonlinear element, which is charged at the rhythm of the addressing cycle. In this cycle, it is of paramount importance that the voltage applied to an addressed image point drops only slightly until the image point is again charged in the next addressing cycle. A quantitative measure of the drop in voltage applied to an image point is the so-called holding ratio (HR) which is defined as the ratio of the drop in voltage across an image point in the nonaddressed state and the voltage applied; a process for determining the HR is given, for example, in Rieger, B. et al., Conference Proceeding der Freiburger Arbeitstagung Flussigkristalle (Freiburg Symposium on Liquid Crystals), Freiburg 1989. Electrooptical systems having a low or relatively low HR show insufficient contrast.
It is true that considerable efforts have already been undertaken hitherto in order to optimize PDLC systems with respect to the liquid crystal mixture used and the polymer system. On the other hand, however, it is still an open problem how to reliably obtain PDLC films with Swiss cheese morphology. Furthermore, only few investigations of PDLC systems having active matrix addressing can be found in the literature, and no concepts have so far been proposed for providing electrooptical systems having
A further serious problem is often that the liquid crystal mixture has insufficient miscability with the monomers, oligomers and/or prepolymers of the polymer used for forming the matrix, which limits in particular the use of PIPS technology in microdroplet matrix systems.
A further disadvantage is in particular that the liquid crystal mixture or individual components of the liquid crystal mixture are in many cases distinguished by an excessively high and/or significantly temperature dependent solubility in the cured, matrix-forming polymer. If, for example, the solubility or the temperature-dependence of the solubility of one or several components differs quite significantly from that of the remaining components, it may happen that the physical properties of the mixture and in particular also of the refractive indices ne and no are substantially affected, which disturbs the adjustment of n.sub.o or n.sub.e or another refractive index of the liquid crystal mixture to n.sub.M, thus resulting in deterioration of the optical properties of the system. The "bleeding" described in EP 0,357,234, according to which at least some of the liquid crystal droplets have the tendency, when the matrix film is subjected to mechanical stress, to dissolve with diffusion of the liquid crystal to the film surface or into the matrix, is favored by a high solubility of the liquid crystal mixture in the cured polymer.
Consequently, there is a high demand for PDLC systems which fulfill to a large extent the requirements described and which reliably exhibit a Swiss cheese morphology. Furthermore, there is a high demand for actively addressed PDLC systems which exhibit a high HR in addition to a Swiss cheese morphology.